Hazardous levels of tiny particulates in smoke from the region's wildfires have prompted warnings from officials that even healthy people should limit their exposure outdoors.

Air quality was listed as "hazardous" in Medford, Grants Pass and Provolt at 5 p.m. Wednesday by the Oregon Department of Environmental Quality.

"Staying inside is really the best, most practical way to reduce your exposure to the smoke," said Jackson Baures, Jackson County public health division manager.

A "hazardous" rating means the air quality index measuring several pollutants is greater than 300. An index of 50 or less is considered "good."

Fine particulates blowing in from wildfires in Douglas and Josephine counties are unhealthy, even for people without respiratory ailments, officials said.

"Those are the particulates that will get deep into people's lungs and cause respiratory problems," said Byron Peterson, DEQ natural resource specialist.

People who must be outside should avoid physical exertion and wear a particulate respirator if possible, Baures said. Masks that help filter out the tiny particulates have the acronym "NIOSH" — National Institute for Occupational Safety and Health — printed on it, along with "N95" or "P100," he said. Wearers should make sure the respirator fits properly and does not leak around the sides.

Dust masks that are not NIOSH-certified may not protect wearers from fine particulate matter, even if they are worn properly, he said.

Even with protection, the masks can actually inhibit breathing among certain individuals with heart-related and respiratory issues.

"It can make it harder to breathe if you have a mask on. That's a factor to consider," Baures said.

Pet owners should also limit outdoor exposure for their animals, make sure they have plenty of fresh water and regularly wash out their eyes and around their mouth with a damp cloth, according to the Jackson County Animal Care and Control Center.

The thick smoke that hung in the air Wednesday prompted several venues to cancel performances, including the Oregon Shakespeare Festival's outdoor Elizabethan Stage, the Ashland City Band and some Medford Parks and Recreation activities.

The smoke is expected to plague Rogue Valley residents for the foreseeable future, National Weather Service officials said.

"Right through next week, other than small changes, temporary changes, there's nothing significant that's going to shift the wind direction," said meteorologist Ryan Sandler. "Any temporary relief would be short-lived."

Some cool air moving through the upper levels of the atmosphere could provide a little reprieve during the daytime hours, but during the night, the smoke will settle back into the valleys, he said.

"It just kind of hangs over us. It's not strong," Sandler said of the upper level flow. "There's no real change in the air mass, that's kind of the key."

The lack of air mixing between the upper and lower levels of the atmosphere is common for this time of year, Sandler said, and the smoke makes any air movement even less likely.

The lingering smoke has set off some fire alarms — including at Asante Rogue Regional Medical Center — causing headaches for local firefighters who must respond.

"It was (Tuesday) evening when the smoke was really bad," Ken Goodson, battalion chief for Medford Fire-Rescue, said early Wednesday. "We've had smoke in the valley before, but I can't say that I really remember that it was setting off alarms."

Reach reporter Ryan Pfeil at 541-776-4468 or by email at rpfeil@mailtribune.com.